1. Field of the Invention
The present invention relates to semiconductor processing, and more particularly to gas and vapor phase cleaning and etching of semiconductor surfaces.
2. Background Information
During the manufacture of semiconductor circuitry, impurities are either purposefully or inadvertently introduced to the semiconductor material. Undesired impurities, also called contamination or microcontamination, may ultimately cause semiconductor device failure. These failures, if occurring at the consumer level, may result in anything from unreliable electronic fuel injection systems to inoperative computer work stations. It is therefore necessary to remove microcontamination from semiconductor substrates during the manufacturing process.
As semiconductor device sizes shrink, the importance of semiconductor substrate cleanliness increases. For example, if a semiconductor transistor has dimensions on the order of 10 microns, as was common about a decade ago, the effect of a 1 micron wide contaminant would not significantly impact the performance of that transistor because the contaminant could only affect a small portion of the total transistor's area. However, if a semiconductor transistor has dimensions on the order of 1 micron or below, as is common today, the same 1 micron wide contaminant could lead to catastrophic failure of that transistor. Extrapolating, it can be seen that high density very-large-scale (VLSI) and ultra-large-scale (ULSI) integrated circuits (IC's), circuits with dimensions below 1 micron, require increasingly stringent levels of reduced microcontamination to achieve reliable device operation.
Metallic contamination, including ionic and elemental impurities, represents a notable threat to the maintenance of strict microcontamination control. This type of microcontamination usually manifests itself as either a surface film, or, if absorbed into the semiconductor substrate, a sub-surface impurity. If metallic contaminants are not removed from semiconductor substrates during the IC manufacturing process, it is likely that the impurities will degrade or inhibit the proper electrical performance and life span of the semiconductor IC's thus increasing the costs while decreasing the quality of consumer electronics.
The RCA clean is the most pervasive and well established silicon substrate cleaning method within the industry today. It consists of two chemical treatments known as SC-1 and SC-2. SC-1 is a basic solution consisting of H.sub.2 O, H.sub.2 O.sub.2, and NH.sub.4 OH. SC-2 is an acidic solution consisting of H.sub.2 O, H.sub.2 O.sub.2, and HCL. The NH.sub.4 OH serves to complex several types of metallic contaminants which are then dissolved into the SC-1 solution. SC-2 then removes alkali ions, cations, and some additional metallic contaminants. Variations on the standard SC-1/SC-2 cleaning method have been developed and automated. Some alternate chemical sequences are SC-1/SC-2/HF, SC-1/HF/SC-2, and H.sub.2 SO.sub.4 -H.sub.2 O.sub.2 /SC-1/HF/SC-2. The RCA cleaning method has proven to be effective in the removal of many species of surface contaminants.
There are several disadvantages, however, to using the RCA cleaning method. One of the main drawbacks is that while the method is useful for removing some forms of microcontamination, other forms of microcontamination are inadvertently introduced to the substrate surface from the wet chemicals used. This is an inherent limitation when working with wet chemical cleaning processes. Even the purest wet chemicals contain significant amounts of contamination in comparison to gases and vapors. In fact, gases and vapors can be two to three orders of magnitude purer than their aqueous chemical counterparts. Incidentally, for the sake of clarity, the noun "gas" and its adjective "gaseous" shall hereinafter be used to additionally encompass the meaning of the noun "vapor" and its adjective "vaporous" respectively.
Another concern in using the RCA cleaning method is the massive amount of chemical waste produced by the wet bench immersion systems used in many automated processing environments. The volume of chemicals required to support these immersion systems adds to the manufacturing costs and poses a significant environmental concern. Automated centrifugal spray cleaning systems using the RCA cleaning method have been developed. These systems reduce the amount of chemical waste produced, but they tend to require considerable maintenance.
Strictly mechanical techniques for cleaning semiconductor substrates such as brush scrubbing, fluid jet, and ultrasonic techniques do little if anything to remove surface films and certainly do nothing to remove sub-surface impurities. These techniques are only useful in the removal of large particles from semiconductor substrate surfaces. In addition, the harsh nature of these mechanical techniques can easily lead to substrate damage.
Choline cleaning chemistry has been proposed as a replacement or modification to the standard RCA cleaning chemistry. Choline, a strong corrosive base consisting of trimethyl-2-hydroxyethyl ammonium hydroxide, appears to be suitable for removing several surface contaminants, particularly when used in place of the NH.sub.4 OH in SC-1. However, as a wet chemical cleaning process, choline cleans suffer many of the same limitations as the RCA clean. One of the most significant limitations is that while some microcontamination is removed from the semiconductor substrate by the choline process, other forms of microcontamination are inadvertently introduced to the substrate surface from the wet chemicals used.
Some dry-cleaning techniques have been developed in order to exploit the purity of gases. One such technique, the UV/CL.sub.2 process, involves using chlorine radicals produced under ultraviolet radiation. This process, however, requires high temperatures on the order of 150.degree.-400.degree. C. for successful removal of metal contaminants. Implementing these high temperatures complicates systems causing them to become more susceptible to breakdown, allows for surface and stub-surface contaminants to potentially diffuse further into the substrate, and contributes to the thermal budget of a process, which is of particular detriment for low thermal budget ULSI manufacturing. In addition, UV/CL.sub.2 processes cannot adequately remove all metallic and ionic contaminants in the gas phase.
Other dry cleaning techniques, in an attempt to keep process temperatures low, typically require larger excitation energies in order to drive the necessary chemical reactions. These excitation energies are typically applied in the form of electromagnetic radiation (for instance, rf radiation to create a plasma). Radiation of this type can have serious detrimental effects on the electrical properties of semiconductor devices. For instance, dielectric breakdown and threshold voltage, shifts are not uncommon results of radiation damage.
These and other cleaning techniques using gas, vapor, or plasma ambients not only require high temperatures but also require very low pressures in order to vaporize the microcontamination. The necessity for very low pressure ambients in such cleaning processes further adds to system cost and complexity as well as contributing to significant increases in throughput time.
Surface micro-roughness is a qualitative description of how rough a semiconductor surface is at the molecular level. There is a strong dependance of thin oxide film quality on the surface micro-roughness upon which the oxide is grown or deposited. Just as lateral dimensions shrink as we enter the ULSI era, so do processes layer thicknesses, particularly gate oxide thicknesses. As oxide layer thicknesses decrease, the oxide layers become more susceptible to electrical breakdown, so the quality and integrity of those layers, and hence the micro-roughness of the semiconductor surface, becomes of paramount importance. Therefore, it is important to consider the effects on surface micro-roughness when designing and implementing a semiconductor cleaning process. Any new semiconductor cleaning process must not significantly roughen the semiconductor surface when compared to the roughening caused by cleaning processes that are currently in use.
In the manufacture of semiconductor devices it is often necessary to implement reactive ion etching (RIE) processes in order to anisotropically etch semiconductor materials. Unfortunately, these processes can severely damage semiconductor surfaces. Typically, these damaged surfaces are removed by plasma etching of the semiconductor material. However, plasma based processes lack the control necessary to etch the semiconductor material to a consistent depth over shallow junctions. This results in unpredictable doping profiles which can cause device failure or significant batch to batch variation in device operation. It would be desirable for a semiconductor cleaning process to be able to remove any RIE damage to semiconductor surfaces with precision and accuracy while smoothing the semiconductor surface.